MECHANICAL PROPERTIES AND HEAT TREATMENT

DESCRIPTION OF PROPERTIES AND THE MECHANISMS BEHIND THE RESULTS.

 

The ISO 8981 standard covers many properties – this course will attempt to explain some of these and their importance to bolted joints. Additionally, heat treatment, the main contributor to the characteristics, will be covered.

 


Mechanical Properties

 
Mechanical properties and markings are governed by standards. There are standardizing organizations around the world, many of them regional or tied to a country or industry. The International Organization for Standardization (ISO) is independent and nongovernmental. ISO published standards are considered the benchmark and often referred to in customer specific standards.

 
The ISO 898-1 Mechanical properties and marking system is one of the main documents in fastener technology. It specifies many of the most important properties of a steel fastener. This course will attempt to explain some of these properties in a graspable manner.


The marking system

 
Bolts and screws

Bulten Academy - Bolts and screws

  1. Manufacturer’s identification mark. Registered trademark.
  2. The first figure denotes one hundredth of the nominal tensile strength of the bolt or screw in MPa. In this case, 100 × 8 = 800 MPa.
  3. The second figure denotes the ratio between the yield strength and tensile strength of the bolt or screw, expressed in tenths. In this case, the ratio is = 0,8. If one multiplies the two figures, the yield stress of the bolt or screw in MPa is obtained. In this case, 800 × 0,8 = 640 MPa.

Nuts

Bulten Academy - Nuts

  1. Manufacturer’s identification mark. Registered trademark.
  2. The figure denotes one hundredth of the tensile strength of the bolt for which the nut can be used without failing. In this case, 100 × 8 = 800 MPa.

 


Tensile strength and Yield strength

 

The tensile strength describes how much load a material can withstand, i.e. its maximum limit. If a material is subjected to a load reaching the tensile limit, it will be permanently deformed. The permanent deformation is proof that the load exceeded the yield strength. A load that is smaller than the yield strength will be in the material’s elastic region, meaning that any deformation will be reversed.

  1. Elastic region, no permanent deformation, the load does not reach yield.
  2. Yield strength, Rp0,2, is an expression for the point where the material begins to permanently deform.
  3. Tensile strength, Rm, is basically the maximum axial strength for a material.

Bulten Academy - Tensile strength and Yield strength

 


Bigger is stronger?

 
Bulten Academy - Bigger is stronger?

One M10 10.9 bolt is strong enough to lift four cars. How many cars can one M12 10.9 lift? Or one M10 B14?

  • Strength is measured by Mega Pascal which is the same as force [N] divided by stress area [mm²], N/mm².
  • A larger dimension will be able to hold a greater force due to bigger stress area.
  • The strength of the bolt is thus dimensionless if the strength of the material is the same.

So which one is stronger? M10 stress area = 58 mm², M12 stress area = 84 mm². 1000 x 58 = 58 kN, 1000 x 84 = 84 kN, 1400 x 58 = 81 kN. The M12 10.9 wins barely against the M10 B14.

 


Hardening of fasteners

To be able to cold-form the wire to a fastener, the wire material must be soft and ductile. However, as mentioned above, our customers demand fasteners with high strength and hardness, therefore we need to harden the fasteners after the cold-forming.

The hardening process is divided into three steps according to the figure below:

Bulten Academy - Hardening of fasteners

1) Austenitization. The fasteners are heated to about 900˚C for about 1 hour in a controlled atmosphere (to avoid decarbonisation). The purpose of this step is to dissolve the carbon locked into the cementite and transform the ferrite to austenite.

2) Rapid cooling. Directly after the first furnace, the fasteners are cooled down rapidly in an oil bath. The purpose of the rapid cooling is to trap the carbon atoms in the iron crystal called martensite. The carbon atoms create disorders in the iron crystals which make the material extremely hard, but also brittle.

3) Tempering. The martensite structure is too brittle to meet the customers’ demands regarding ductility. Therefore the hardening process is followed by a tempering process which makes the fasteners more ductile but also lowers their strength. By using different temperatures and time during the tempering process, the same wire material can be used for 8.8, 10.9, and 12.9 screws. During the tempering process, the carbon atoms are released and react with iron atoms. After the tempering, the structure is tempered martensite with small islands of iron carbides.


Diploma - Mechanical Properties

Learn more – take the full training

 
This is a shortened version of the Bulten Academy training course Mechanical Properties. To take part of the full version, please contact Bulten Academy – academy@bulten.com.